Fan Header Voltage Questions

[macrospect]

Hello,

Its been a while since I have posted in the electronics forum. I came here because I am seeking some assistance on how to wire up a flow meter. First let me describe what I am trying to accomplish. I have many ideas but do not know how I am going to accomplish them!

I recently purchased a Proteus 205 flow meter for my waster cooling setup. Since I have nearly a grand in components that is being cooled via water cooling I wanted to add a failsafe incase something catastrophic happened while I am not home and my computer is on. What this flowmeter does is it uses a spinning impeller to create a 5VDC signal. At first I was going to create a relay which is hooked up to the PSU in my tower. If that tripped then the computer would shut off. Then I thought I would need some sort of a time delay because the relay would be in a tripped state until my loops GPM's increased as my computer first started up.

Since that seemed a little too complex I later thought of option B; fan headers! The only problem is I have NO clue how a fan header works (voltage wise). I tested one of the fans in my room with a DMM and saw ~200-300mV VAC. When I stuck my finger into the fan to stop it the mV shot above 300 and that created an open circuit. Now, the problem is I need to take a 5VDC analog signal and transform it into a AC signal which is only a few mV.

Is this possible or just wishful thinking?

I really appreciate any help! Thanks!

 

[aL Mac]

I think what you are trying to do by creating a fail safe is a cool idea.

However, I'm not sure what you are trying to do with the motherboard fan header. The motherboard headers supply DC voltage not AC. 12 volts. Even so, I'm not sure how you would plan on using the motherboard header. Also I bet the impeller's voltage is not a constant 5V but varies depending on the amount of water flow.

You should post some pictures or a link to this flow meter and more details and maybe we can help you design a circuit or something that could help.

 

[macrospect]

Hello aL Mac,

Thanks for the reply. Sorry I was a bit sketchy on the details. Yes, this flow meter does vary in output voltage (from 0-5V depending on the amount of flow). The meter itself has 4 wires which come out of it. Two are for the 24VDC supply current and the other 2 are for the 0-5VDC output signal. I know PSU's don't have a 24V supply on them so I was just going to use a little transformer to crank 120V down to 24V.

Now, for the fan headers. I know there are commonly 3 pins on a fan header. 1 is 12V, 1 is ground, and the last is for a RPM cable. I am pretty sure if I shoved 5V into that RPM pin that I would destroy my mainboard. So, what I wanted to do is figure out if I could possibly step the voltage down to a safe level and have my BIOS monitor the "RPM's" coming from the pump. I am not really interested in having the RPM's being accurate. I just want some kind of a constant reading going to the mobo (read on). Since I can configure my BIOS to shut the computer down in case of a fan failure it would be perfect if I could hook up the flow meter to a fan header. If it fails then my computer would shut down before any damage is done.

If you need some specs then check Proteus's site out:
http://www.proteusind.com/200/200_flow2.html

There they have installation manuals and product manuals. Personally, I have the 205 series. Ignore what some of the other manuals say. I only have 4 wires coming out of mine. I described exactly what they are used for above.

Again, it seems like a simple concept but I have no idea how I would go about doing this.


Thanks again for the reply!

 

[cpemma]

...this flow meter does vary in output voltage (from 0-5V depending on the amount of flow

So, what I wanted to do is figure out if I could possibly step the voltage down to a safe level and have my BIOS monitor the "RPM's" coming from the pump.!

The pump puts out a variable DC voltage, but the fan header RPM pin is designed to take 0V pulses. It's normally pulled high to either +5V or +3.3V, depends on the chipset, a PC fan's electronics grounds the pin twice per rev and the chipset firmware "counts up" in the interval between pulses and computes an RPM.

To interface your flowmeter you need a voltage-to-frequency converter to get a variable reading. An easier way is to just tell the computer the flow has dropped below a set level, using the fan failure alarm to then make things safe - if the header pin receives no pulse before the above count "times-out"(actually, if it gets up to 255) it triggers the alarm. For that you could use a 555 to generate a fake pulse train enabled or disabled by a comparator switch on the flow voltage.

 

[macrospect]

Hello cpemma. Wow! That is a lot of helpful information. I tried to search Google for some schematics and such.

From what I gather a 555 generator is a simple circuit which uses different micro farad caps to open or close the circuit. If the input voltage dies and the caps drain then the circuit will open. Is that about right? here is a schematic that I found: http://casemods.pointofnoreturn.org/pwm/555.html

Does that mean I will need to purchase a 555 chip?

As for the comparator switch that goes before the 555 generator, correct? Is this an example of a comparator switch? Looks really easy to build if so!

http://www.mitedu.freeserve.co.uk/Circuits/Switching/comparator.htm

Now, the 555 generator is only sending a steady signal to my mobo right? If that signal fails to "pulse" then my computer will shutdown.

Lastly, is this something I can purchase or would that be totally pointless. I do have some experience with breadboard so its not out of the question to go and build something like the 555 and comparator switch.

Sorry I couldn't better understand. I am new to these components!

Thanks for your helpful reply too!

 

[Mohonri]

Does that mean I will need to purchase a 555 chip?

As for the comparator switch that goes before the 555 generator, correct? Is this an example of a comparator switch? Looks really easy to build if so!

http://www.mitedu.freeserve.co.uk/Circuits/Switching/comparator.htm

Now, the 555 generator is only sending a steady signal to my mobo right? If that signal fails to "pulse" then my computer will shutdown.

Lastly, is this something I can purchase or would that be totally pointless. I do have some experience with breadboard so its not out of the question to go and build something like the 555 and comparator switch.

A voltage comparator can be even simpler than the schematic you linked. All you need is a simple op-amp, powered off 12V. Make a voltage divider using a couple of resistors to get a reference voltage of somewhere 2-3V. Connect that to the + input on the op-amp. Connect the voltage from the flow meter to the - input. Then connect the output of the op-amp into the reset pin of the 555 chip.

Here's what happens--under normal circumstances, the voltage from the flow meter will be at 5V, with a reference voltage of 2 (or 3) volts. So the comparator will see the + pin at a lower voltage than the - pin, and drive its output low. When the reset pin on the 555 is held low, the timer operates normally, sending pulses out at whatever interval you specify with the resistor(s) and capacitor you attach. When the flow stops, the input to the comparator will fall to 0, so now the + pin will be higher than the - pin, so it will drive its output high, which holds the reset pin on the 555 high. IIRC, a 555 will hold its output high when its reset pin is held high, so you will have no pulses coming out of the 555. The mobo will see that and shut down the computer.

Does that make sense?

 

[macrospect]

I fully understand Mohonri. Thanks for explaining that for me.

Now I need to figure out how to build a operational amp. I am trying to learn a little about them. If you have any good links to some simple schmeatics please feel free to share!

Oh and I forgot to mention a few things earlier. This flow meter is made for 4GPM max. Proteus says if you exceed that it may shorten the life of the meter. With my system I get around 6 GPM. However, that is with perfect flow (basically with no head whatsoever on the pump). I was going to measure my exact gpm to see where I am at. I am hoping I am near 4 but I am not sure at this point.

Lastly the flow meter has a little pot on it to adjust the output voltage. From what I gather the voltage coming off of the flow meter should be somewhere from 4-5V. Again, that can be adjusted.

This project isn't looking as complex as I thought!

 

[Mohonri]

You don't have to build an op-amp. They're prepackaged IC's. In fact, on second thought, you don't even need an op-amp. Just an n-channel MOSFET or NPN BJT, set up as a common-emitter (or common-source for MOSFET) amplifier. Link

 

[macrospect]

Thanks for clarifying that for me Mohonri. Since this is an integrated IC that we are talking about does that mean that a LM1458 would be an example of an operational amp? If something like that would not work as an op-amp then could you please provide me with a number of something which would?

Thanks again for your help!

 

[Mohonri]

Yeah, a LM1458 would work just fine.

 

[cpemma]

LM1458 (and the 741) has one snag, on a single-rail supply (like the 12V in a PC) the output won't go low enough when "off" to turn off a transistor (ie, below about 0.6V). There are work-rounds, but LM358 is a cheap & common op-amp that's designed for single-rail supplies and will swing down to 0V.

The 555 should drive an 'open-collector' NPN transistor to mimic a fan speed wire. That's 555 pin #3 through a 1k-10k resistor to the transistor base, transistor emitter to ground, and transistor collector to the speed pin on the mobo. Like fig.3c here.

 

[Rondu]

A simpler solution may well be to design voltage to frequency circuit (as cpemma mentioned in his first post) using an LM331. You need only a single IC a couple of capacitors and a few resistors, it already has an open-collector output. You also will get the "benefit" of being able to monitor the actual flow rate.

 

[macrospect]

Thanks for more replies! I have not bought any ic's yet because I am still waiting for input. Like I said in my first pots I wanted to hear a the various ways that I can accomplish this.

Rondu, since you are saying a LM331 will do, does that mean that I will no longer need an OP amp?

I found this schematic of the LM331. Let me know if the circuit I need to build is similar to the one found in this PDF file:

Linky

 

[Rondu]

DaRkF0g - use the circuit shown in Fig 3 in the application notes (Simple Stand-Alone V-to-F Converter). For your purposes you can leave out the offset adjust and you do not need the 10k pull-up resistor (connected to Vlogic), motherboards have this pull-up resistor built-in. To get a reading of 0-2500 rpm for 0-5V input use a 14.3 k resistor for Rs. Your flowmeter is rated at 0-5V output for 0-2.5 gpm flow. You don't need the op amp just the LM331, five resistors (use 1% metal film) and 3 capacitors.

 

[macrospect]

Thanks for clearing that up for me Rondu.

From what I understand the LM331 has eight pins on it:

-Vcc
-Comparator Input
-Threshold
-R-C
-Current Output
-Reference Current
-Frequency Output
-Gnd

Ok, so those are my eight pins. I am still a bit confused about some of them. I do not know what Vcc is (is that my 12V input from the pc?). I do not know the difference between the comparator input and the threshold. I thought the comparator was providing the threshold value (which brings up the point, I still need to build a comparator, correct?). I am not sure what R-C is or if I need that. Current output and Frequency output- would I just be using frequency output since it is a frequency I want going to the fan header. Reference current- that’s what’s coming from the pump correct? Gnd is obvious.

Do I have anywhere near the correct idea?

Those diagrams in Figure three are confusing me a bit. I know the pump sensor essentially goes where the fan is but where does the LM331 come in at?

As for the five resistors I am guessing I need to figure out their values? Are they connected in a series or to different pins on the LM331?

Sorry to be so clueless. All of this with IC's is brand new to me. I much appreciate your help!

 

[Rondu]

Sorry my earlier post wasn't clear, you can get the Application Notes for the LM331 from National Semiconductor at http://www.national.com/ds/LM/LM231.pdf . On page 8 of the Application Notes there is a sample circuit for a simple V-to-F converter (fig 3) generally all of the component values on this circuit will work for your purposes, in my last post I suggested a few changes to simplify this further as you do not need high precision.
Vin is the output from your flow meter, Vs is 12V from your computer power supply and Fout connects to one of the fan speed sensor inputs on your motherboard. Those are the only connections you need other than grounds to the motherboard and sensor. Hope that explains it more clearly.

 

[macrospect]

Thats much better. Thanks for clarifying that for me. I now fully understand what is going on. Thanks for the detailed schematic too. That was exactly what I was looking for! I am sure I will be back after I get all of my parts in!

Thanks for all of your time and help Rondu (and everyone else as well)!